The present invention relates to a rocker arm for use in an internal combustion engine, and more particularly, to the rocker arm provided with a cast iron member having high wear resistance and scuffing resistance at least at a pad or abutting portion. The pad portion may be an upper rocker arm portion in sliding contact with a rotating cam lobe of a camshaft or a lower tip end portion in abutment with an upper valve stem end or in slide contact with a rotating cam lobe.
As shown in FIGS. 1 and 2, there are several types of rocker arms, such as a pivot type rocker arm (FIG. 1) and a rocker shaft type rocker arm (FIG. 2). The former type is generally adopted in an OHC engine, and in which a rocker arm body 1 has one end 1a pivotally supported to a spherical portion of a pivot member 2, and another end portion 1b provided with a lower surface portion 1d (pad portion) in abutment with an upper end of a valve stem 3. A cam lobe 4a of a camshaft 4 is in rotational contact with an upper surface portion (another pad portion) 1c of the rocker arm body 1, so that the lower surface portion 1d urges the valve stem 3 in its axial direction upon rotation of the cam lobe 4a. In case of the rocker shaft type shown in FIG. 2, a rocker arm 1' is pivotally supported by a rocker shaft 5. One end (pad portion) 1a' of the rocker arm is in contact with a rotating cam lobe 4a, while another end portion (another pad portion) 1b' is in contact with an upper valve stem end 3. Upon rotation of the cam lobe 4, the rocker arm body 1a' is pivotted about the rocker shaft 5 to urge the valve stem 3.
With the structure, the pad portions 1c, 1d and 1a' of the rocker arms abut these opponent components such as the cam lobe 4a and the valve stem 3 at relatively high pressure. Therefore, these portions must provide high wear resistance. In this connection, conventionally, the rocker arm body in its entirety or at least these pad portions thereof is formed of chilled iron, cast steel or sintered alloy.
In case the sintered alloy chip is incorporated to the pad portions, it is fixed to the rocker arm body formed of cold forging steel by brazing or burning. However, such unitary structure lead to high production cost. Further, if the chilled iron or cast steel chip is used as a wear resistant chip member at the abutting portions, the chip undergoes excessive frictional wear in use under high performance high load engines those being up-to-date demands.
On the other hand, known is 28%Cr cast iron which exhibits sufficient wear resistant property because of its inclusion of carbide having high hardness such as (Cr.Fe).sub.7 C.sub.3. However, in this cast iron, ferrite may be precipitated, and therefore, sufficient wear resistivity would not be obtainable if such cast iron material is used in the rocker arm which is operated in association with a high speed high load engine.
Japanese laid open patent application, Kokai No. 56-129710 discloses a rocker arm in which several portions in abutment with the camshaft and the valve stem are formed of cast iron containing from 5 to 36 % of Cr, and abutting surfaces of the cast iron piece are subjected to nitriding treatment. The present invention is an improvement on the rocker arm abutting portion by providing specific combinations of compositions and their percentages taking also metallic structure of the cast iron into consideration.